1. Field of the Invention
The present invention relates to a method and apparatus for stranding tape-shaped optical fibers in lamination each of which includes a plurality of optical fibers arranges into a tape-shaped optical fiber.
2. Description of the Related Art
FIG. 4 is a cross section showing an example of a tape-shaped optical fiber. In FIG. 4, the tape-shaped optical fiber 1 includes a plurality of coated optical fibers 2 which are arranged in parallel to each other and the whole outside of which are covered with a coating layer 3 so that the coated optical fibers 2 are integrated into a unit as a tape-shaped optical fiber. Generally, an UV cure acrylate resin is used as the coating layer 3.
FIG. 5 is a cross section of an example of a slot cable using the tape-shaped optical fibers as mentioned above. As shown in the drawing, the optical cable includes a slotted spacer 4 having a tensile member 5 at central position thereof and a plurality of optical fiber storing slots 6A, 6B, 6C and 6D which are spiral grooves at its circumference. In each of the optical fiber storing slots 6A, 6B, 6C and 6D, a plurality of tape-shaped optical fibers 1 are stranded and stored in lamination. The outer circumference of the slotted spacer 4 is covered with an wrapping tape 7, and the outer circumference of the wrapping tape 7 is further covered with an outer sheath 8 of polyvinyl chloride, polyethylene, or the like.
When a plurality of tape-shaped optical fibers are stranded in lamination as described above, proper excess length cannot be given to the tape-shaped optical fibers because of the high friction coefficient between the respective UV cure acrylate resins applied to the outsides thereof. For such a reason, the transmission loss of the optical fibers become abnormal, and in the case where a cable is bent after the constitution thereof, the tape-shaped optical fibers become difficult to move after the stranding so that the tape-shaped optical fiber located on the outside of the bending is subjected to strain beyond an allowable limit. Accordingly, the reliability is deteriorated and particularly the lifetime till break is shortened. Therefore, in order to reduce the friction between the tape-shaped optical fibers, generally, powder, such as talc powder, or the like, giving sliding property is applied to the tape-shaped optical fibers.
It has been general conventionally that such powder was applied in a period from the formation of tape-shaped optical fibers to the taking up of the tape-shaped optical fibers onto bobbins or the like in the tape-shaped optical fiber manufacturing process or applied while the tape-shaped optical fibers were wound up again in a separate step after production of the tape-shaped optical fibers.
In such a conventional powder application method as mentioned above, the friction coefficient between the tape-shaped optical fibers becomes low by the application of the powder. Accordingly, in carrying the tape-shaped optical fibers after wound-up into a stranding step, stranding the tape-shaped optical fibers or the like, the tape-shaped optical fibers may easily get out of their winding shape in winding up of the tape-shaped optical fibers onto bobbins or the like after application of powder. Further, proper tension control thereof becomes difficult in the stranding of the tape-shaped optical fibers or the tape-shaped optical fibers, and the tape-shaped optical fibers become impossible to be fed out because getting out of their winding shape so that they may be broken.
Further, as for the conventional method or apparatus for applying powder to the tape-shaped optical fibers, it is difficult to apply the powder uniformly in the longitudinal direction of the tape-shaped optical fibers, because the tape-shaped optical fibers are simply passed through the powder. If the powder does not apply-uniformly on the surface of the tape-shaped optical fiber, the frictional coefficients of the tape-shaped optical fiber become different at various portions thereof. Accordingly, when the optical cable in which a plurality of tape-shaped optical fibers are stored in the slots is bent, some of the tape-shaped optical fibers may be undulated in the slots.